The eldritch invocation Shroud of Shadow from the UA: Revised Class Options reads as follows:
You can cast invisibility at will, without expending a spell slot.
Similar abilities such as a wizard's spell mastery specify that the caster can only cast the at-will spells at their lowest spell level:
You can cast those [at will] spells at their lowest level without expending a spell slot when you have them prepared.
Shroud of Shadow has no such clause. So can this invisibility spell be cast at a higher level?